When you get it out of storage, take a good look under there. Park on a hard, level surface. Then block the rear wheels in front and behind both sides. Put it in Neutral, Parking Brake OFF. This'll free the transmission end of the shaft so you can try to find looseness.
Inspect the shafts. Look for dents, bends, and anything stuck to them or wrapped around. Push and pull on the carrier bearings between sections. This won't do much to help identify stuck U-Joints, only ones that are falling apart.
Driveshaft work isn't terribly expensive. At least if you pull the shaft and take it to the shop. Call around or visit several shops.
If you DO take it out, mark how the carrier bearing fasteners sit. Ask the shop how it should line up. I think they'll tell you there should be the little offsets I mentioned earlier.